The present invention relates to the field of celestial tracking apparatuses. More specifically, the present invention relates to the field of wind stow of celestial tracking apparatuses.
Celestial tracking apparatuses are devices that track or face a given object in the celestial hemisphere during normal operations. Celestial tracking apparatuses are typically configured for use as radio telescopes, radar systems, solar collectors, etc.
The object being tracked is typically moving relative to the surface of the Earth, and the celestial tracking apparatus must be able to accurately track the object.
A celestial tracking apparatus may be large. For example, the Lovell radio telescope at the Jodrell Bank Observatory of the University of Manchester at Macclesfield, Cheshire, United Kingdom, has a dish diameter of 76.2 m (approximately 250 ft.), resulting in a collection area of over 4560 m2 (approximately 49,100 sq.ft.). More directly, in an exemplary photovoltaic (PV) solar power collection unit used throughout this discussion, the collector is a substantially flat PV array having an approximate frontal surface of 13.6xc3x9715.8 m (approximately 44.6xc3x9751.8 ft.), resulting in a collection area in excess of 214 m2 (approximately 2,300 sq.ft.).
Celestial tracking assemblies presenting large collection areas to the wind are subject to considerable wind stresses. When the wind exceeds a given speed, these stresses may become destructive. The celestial tracking assemblies are therefore configured to assume a predetermined attitude when the wind acting upon them exceeds a predetermined excessive wind speed. This attitude is known as wind stow.
Several problems exist with conventional methods and structures for placing tracking apparatuses into wind stow. All such methods and structures involve compromises in cost, maximum apparatus size, reliability, and safety.
When in wind stow, the collector (i.e., the movable portion of the celestial tracking apparatus that actually faces or points to the celestial object) is positioned to minimize the effects of the wind. This is accomplished in several ways.
When in wind stow where the accumulation of ice and snow may be a problem, the collector is often positioned to face downwind substantially horizontally. This allows the collector to receive the wind at its back, where structural members may be positioned to absorb the wind-induced stresses without interfering with normal operation. In many cases, a smaller collector is configured to xe2x80x9cweathervane,xe2x80x9d i.e., to freely pivot azimuthally so that the collector may maintain its downwind position as the wind shifts.
When in wind stow where the accumulation of ice and snow are not a consideration, as for a solar power collection unit located in a desert environment, the collector is often positioned to point vertically, i.e., at the zenith. When pointing vertically, the collector itself is substantially horizontal (parallel to the ground) and less affected by substantially horizontal winds.
A vertically pointing wind-stow position is suitable for a truly horizontal wind, as the collector then presents a symmetrical edge regardless of wind direction. A problem exists, however, in that a wind is typically only approximately horizontal, and often has an upward or downward component. This upward or downward component is usually the result of wind movement over the nearby terrain (hills, cliffs, etc.) or obstructions (buildings, walls, etc.).
Unless, the upward or downward wind component is severe, (e.g., greater than 15xc2x0), a vertically pointing wind-stow position remains suitable for a dish-type collector. Such a collector presents a symmetrical edge to a substantially horizontal wind from any direction.
For a substantially horizontal flat collector (i.e., a collector pointing at the zenith), a problem exists in that the presence of even a small upward or downward component in the wind interacts with the collector to produce an airfoil effect. This airfoil effect produces a force, lift, which acts substantially perpendicularly to the wind. With an anterior or posterior wind, i.e., a substantially horizontal wind substantially perpendicular to the axis of the elevation pivot, this force would attempt to drive the collector out of wind stow. This places an additional burden upon the elevation pivot and actuators coupling the collector assembly to the rest of the celestial tracking apparatus.
Another problem exists in that a potentially destructive wind may occur rapidly. This is exemplified by the haboobs that occur in the subtropical desert regions worldwide. Such a haboob may cause a shift in wind speed from less than 4.5 m/s (approximately 10 mph) to greater than 25 m/s (approximately 56 mph) in less than 30 s. Conventional celestial tracking apparatuses typically take several minutes to shift from normal operation into wind stow. The transition from normal operation occurs far too slowly to provide adequate protection against the onset of a severe haboob. Unfortunately, those locations that are ideal for solar collectors, the subtropical deserts, are also those locations most prone to haboobs and other abrupt wind phenomena.
Rapid wind stow, while desirable, creates an additional problem. Wind stow is desirably performed automatically. That is, the celestial tracking apparatus desirably should itself detect the presence of a potentially damaging wind and place itself into wind stow without human intervention. Therefore, if a celestial tracking apparatus were to be built that could place itself into wind stow rapidly enough to handle the onset of a haboob, that celestial tracking apparatus would then pose a hazard to maintenance personnel. For example, an individual may be trapped and crushed by a rapidly descending collector assembly. It is therefore desirable for maintenance personnel to be able to temporarily disable automatic wind stow.
It is desirable that a celestial tracking apparatus automatically enter wind stow in response to wind exceeding a predetermined excessive wind speed for a predetermined length of time. It is also desirable that the celestial tracking apparatus automatically exit wind stow when the wind has subsided, i.e., when the wind is less than a second predetermined safe wind speed for a second predetermined length of time.
When in wind stow, the collector assembly is placed in a safe position. That is, a celestial tracking apparatus in wind stow is prepared for adverse weather, etc. This makes it desirable that an inoperative celestial tracking apparatus be placed in wind stow. Desirably, a system failure would cause the system to default into wind stow, i.e., the system would be xe2x80x9cfail-safexe2x80x9d for wind stow. xe2x80x9cFail-safe,xe2x80x9d as used herein, is taken to mean xe2x80x9cequipped with a secondary system that insures continued operation even if the primary system fails,xe2x80x9d Random House Webster""s Unabridged Electronic Dictionary, copyright (copyright) 1996 by Random House, Inc. For example, were the electrical power to fail because-of an advancing storm, the celestial tracking apparatus desirably has some means of automatically reverting to wind stow without electric power so as to prevent damage to the celestial tracking apparatus upon arrival of the storm. This presents a problem for conventional wind-stow methods as structures, where power is required to place the apparatus into wind stow. Typical solutions are batteries and/or auxiliary generators, which add to the cost and complexity while decreasing decrease the reliability of the apparatus.
Since a celestial tracking apparatus in wind stow is prepared for adverse weather, etc., it is desirable that wind stow be capable of being maintained indefinitely on demand. That is, a celestial tracking apparatus intentionally placed in wind stow should desirably remain in wind stow, without application of power or control, until intentionally released from wind stow. This allows celestial tracking apparatuses taken out of service for extended periods to be protected against adverse weather.
Accordingly, it is an advantage of the present invention that a celestial tracking apparatus and method of controlling wind stow therefor is provided.
It is another advantage of the present invention that a method is provided for automatically placing a celestial tracking apparatus into wind stow when a wind exceeds a predetermined excessive wind speed for a predetermined length of time.
It is another advantage of the present invention that a method is provided for automatically removing a celestial tracking apparatus from wind stow when a wind has abated below a predetermined safe wind speed for a predetermined length of time.
It is another advantage of the present invention that a method is provided for automatically placing a celestial tracking apparatus into wind stow upon occurrence of a system failure.
It is another advantage of the present invention that a method is provided for directly placing a celestial tracking apparatus into wind stow.
It is another advantage of the present invention that a method is provided for indefinitely retaining a celestial tracking apparatus in wind stow.
It is another advantage of the present invention that a method is provided for inhibiting a celestial tracking apparatus from entering wind stow.
The above and other advantages of the present invention are carried out in one form by a celestial tracking apparatus formed of a support, a tracking assembly, a first pivot coupled between the support and the tracking assembly, a collector assembly, and a second pivot coupled between the tracking assembly and the collector assembly. The collector assembly has a center of gravity, a facing plane, and a target axis substantially perpendicular to the facing plane and passing through the center of gravity. The second pivot is displaced from the target axis and the center of gravity thereupon.
The above and other advantages of the present invention are carried out in another form by a method of controlling the placement of a collector assembly of a celestial tracking apparatus into a wind-stow position. The collector assembly has a center of gravity, a facing plane, and a target axis substantially perpendicular to the facing plane and passing through the center of gravity. The method includes pivoting the collector assembly about a pivot displaced from the target axis and the center of gravity.